Heat exchangers often include internal fluid flow passages in which the fluid must change direction at least once as it flows between an inlet and an outlet. For example, compact heat exchanger designs often place the inlet and outlet at a first end of the heat exchanger. A rib is located between the inlet and outlet and extends to a point which is close to the second end of the heat exchanger, to prevent short-circuiting of the fluid flow. The fluid is forced to flow through a gap between the terminal end of the rib and the second end of the heat exchanger, and undergoes a change in direction of 180 degrees. The fluid therefore follows a U-shaped flow path and makes two passes along the length of the plate. Examples of compact heat exchangers are described in U.S. patent application Ser. No. 14/188,070 (published as US 2014/0238641 A1), and U.S. patent application Ser. No. 13/599,339 (published as US 2013/0061584 A1), both of which are incorporated herein by reference in their entireties.
Imposing a change in the direction of an internal flow field often leads to separation of the boundary layer from the adjacent wall. The flow separation results from the presence of an adverse pressure gradient strong enough to overcome that imposed by frictional losses at the wall, causing the fluid in the boundary layer to reverse direction. Once a favorable gradient is restored, the flow may reattach to the wall, creating a zone of stagnation or low velocity recirculating flow referred to as a separation bubble. This zone is often called the wake or a dead zone.
From a design perspective, it must be realized that not all curved flows result in a local adverse pressure gradient large enough to induce flow separation. The tendency of a flow to separate is a function of the radius of curvature of the adjacent surface, the viscosity of the fluid, and the velocity of the fluid (i.e. the Reynolds Number). According to Bernoulli's principle, when a streamline is exposed to a rapid increase in flow area, such as that associated with a very small radius of curvature, the local velocity decreases sharply, in turn significantly increasing the local hydrostatic pressure and causing the flow to separate. Increasing the radius of curvature by increasing the width of the rib is not an attractive option since a wider rib will reduce the heat transfer area.
FIG. 22 shows an example of a standard U-flow core plate design with a central rib of small radius, illustrating the separation of flow along the rib immediately downstream of the point at which the fluid flow changes direction. The approximate area of flow separation is the lined area which is enclosed by dotted lines.
An example of a heat exchanger where very high wall temperatures could be expected is an Exhaust Gas Heat Recovery (EGHR) heat exchanger. The core of an EGHR heat exchanger typically comprises a plurality of flow passages for flow of a liquid coolant and a plurality of flow passages for flow of a hot exhaust gas, the coolant and exhaust flow passages alternating throughout the core structure and being defined by a stack of core plates. Heat transfer from the exhaust gas to the coolant may be enhanced by placing turbulence-enhancing inserts within the exhaust flow passages, where each insert may be bonded to the plates of the core stack along its top and bottom surfaces.
Where the EGHR heat exchanger includes U-shaped or serpentine flow passages for the coolant, the presence of dead zones not only degrades the overall heat transfer coefficient, but also increases the risk that a water-containing coolant circulating through the heat exchanger can boil. Where the fluid circulating through the heat exchanger is transmission fluid or engine oil, it is possible for the fluid to become overheated to the point that coking will occur in these dead zones.
Increasing the width of the rib in such an EGHR heat exchanger will lead to a decrease in the heat transfer area in the coolant flow passages, due to the space occupied by the rib. In the exhaust flow passages, the core plates will be unbonded and out of contact with the turbulence-enhancing inserts in the area of the rib, and therefore widening the rib will similarly reduce the heat transfer area in the exhaust flow passages. The inclusion of additional ribs and dimples in the coolant flow passages will have a similar negative effect on the heat transfer area in the coolant and exhaust flow passages.
There remains a need for a heat exchanger structure which will avoid the formation of dead zones under a range of operating conditions.